Last year, staff at U.S. consumer rights advocacy group Public Citizen sent a report to the FTC accusing stars including Kim Kardashian, Rihanna and the boyband One Direction of promoting products online without disclosing whether they were being paid to do so.

A statement published on the FTC's website reads, "After reviewing numerous Instagram posts by celebrities, athletes, and other influencers, Federal Trade Commission staff recently sent out more than 90 letters reminding influencers and marketers that influencers should clearly and conspicuously disclose their relationships to brands when promoting or endorsing products through social media."

The letter sent to celebrities highlights specific cases where they may have published social media posts praising a brand without making clear they had received free products or services or were being paid for their endorsement.

Researchers with Public Citizen's investigation alleged that 113 celebrities published endorsements on Instagram without proper disclosure.

In a statement reacting to the FTC's written warning, the group's president said, "The FTC has validated our concerns, sending 90 letters to influencers and also to the brands that employ them, informing them that their practices are in violation of FTC guidelines. This move is welcome, but insufficient."

Calling on the FTC to go after stars who broke the rules he added, "Ideally, the FTC should announce that it is investigating repeat offenders and quickly bring action against them."